To profile metal tubes of given length and cross section, various methods are used to convert the original cross section of the tube to a different, e.g., circular, square, rectangular, lobed, star-shaped, cross section, etc.
One of the most common methods is to feed the tube through a number of forming dies aligned in a given travelling direction of the tube and each comprising a number of rolls arranged to define a passage for the tube.
The cross sections of the successive passages differ from one another, and increasingly approximate, in the travelling direction of the tube, the final cross section of the tube, so that the tube, as it proceeds in the travelling direction, is gradually deformed from its original to the desired final cross section.
The above method produces profiles of fairly good quality, but has several drawbacks which may seriously impair output.
A first of these lies in anomalous deformation of the leading end portion of the tube when the tube is inserted between the rolls of the dies. As a result, the end portion typically must be removed at the end of the profiling process, thus resulting in additional cost in terms of both equipment and waste.
Another drawback of the above method derives from the fact that the forming dies are normally designed for a given tube size and a given final cross section, so that, for each different starting size of the tube and/or each different final cross section, all or some of the dies typically must be changed, thus incurring additional cost in terms of production holdups and the high cost of the equipment required.
To eliminate the latter drawback, which may get worse as the tube gets bigger, a different method has been proposed whereby all the dies, or at least all those interposed between an initial rough die and a final finish die, are replaced by a number of pairs of opposite rolls movable, with respect to each other and within a given range, in a radial direction with respect to the tube axis.
Though more flexible, by being fairly adaptable to the size and shape of the tubes, this solution fails to solve the first of the drawbacks described above, relative to anomalous deformation of the leading end of the tube.
A solution to this problem is proposed by WO-A-2008/022626, which is incorporated by reference and which teaches to feed a tube between a pair of spaced apart rolls, which are then closed onto an intermediate portion of the tube and set at a distance to one another less than the external diameter of the tube, which is heated in order to allow radial penetration of the rolls. The tube is then reciprocated between the rolls to obtain deformation of the aforementioned intermediate portion of the tube. The final shape of the tube is obtained by adjusting the gap between the rolls in a stepped manner.
The above solution suffers from a number of drawbacks mainly because the radial load applied by the rolls to the tube at any step-adjustment of the gap is a static radial load, which would involve ovalization of the tube should the tube not be heated. Moreover, the axial forces necessary to start moving the tube axially are so high that the transverse stability of the rolls is typically always put in jeopardy.